Pitching moment recorder



INVENTORS AGENT ANTONR. HOFFMANNR WALTER J. CRUMP A. R. HOFFMANN ETALPITCHING MOMENT RECORDER 1 Filed Jan. 29, 1946 STICK LOCKED TAB ADJUSTEDNO RESULTING MOMENT STICK FREE March 10, 1953 NEG POS

Patented Mar. 10, 1953 PITCHING MOMENT RECORDER Anton R. Hoffmann,Southport, and Walter J. Cramp, Stratford, Conn., assignors to UnitedAircraft Corporation, East Hartford, Conn., a

corporation of Delaware Application January 29, 1946, Serial No. 644,016

'5 Claims. 1

This invention relates to means for measuring the moments of a rotorblade or blades for helicopters, or the like, and for adjusting theblade or blades in accordance with such measurement.

In helicopters, during flight, certain vibrations may be present due tounbalance of the rotor blades with respect to each other. Suchvibrations will cause an unbalanced resultant moment acting on thecontrol system felt in the control stick as a one per revolution shakeand may render the controls inaccurate or difficult to operate orfatiguing to the operator.

Hitherto, devices have been built to measure such vibrations so thatcorrections could be made in the rotor blades therefor. Such deviceshave included synchroscopes and electrical recorders and have been bulkyand diiii-cult of manipulation, and not suited for use away fromfactories or completely equipped airport services.

It is an object of this invention to provide a simplified, but veryaccurate, means for balancing rotor blades for helicopters, or the like.

Another object is to provide a device as set forth in the above objectthat may be readily installed and used by a person not necessarilyskilled in the measuring arts.

Other objects reside in the details of construction, arrangement ofparts, and will be either obvious or pointed out in the followingspecification and claims, in view of the drawing of a presentlypreferred embodiment, in which:

Fig. l is a diagrammatic plan view of a threebladed rotor type ofhelicopter;

Fig. 2 is an elevational view *with parts broken away and non-essentialparts omitted for clarity;

Fig. 3 is a diagrammatic view of the chart and stylus arrangements;

Figs. 4: and 5 are sample charts; and

Fig. 6 is a diagrammatic illustration of a modified form of theindicating mechanism of Figs. 1 through 3.

Referring first to Figs. 1 and 2, a helicopter has a body ill in whichan engine 12 is mounted. The engine l2 turns a drive shaft 14 throughsuitable reduction gearing Hi to turn several rotor blades [8 carriedabove the body 10. The rotor blades are mounted upon the drive shaft Mby universal hinges, not shown, but which may be substantially the sameas those shown in application Ser. No. 592,862 of Igor I. Sikorsky,filed May 9, 1945, now Patent No. 2,517,509 of August 1951. The rotorblades are equipped with tabs 26 which may be bent by suitable toolsbetween limits indicated at 22, Fig. 2, to change the pitching moment ofa blade due to aerodynamic forces.

Each of the rotor blades 18 is provided with a control arm. 24 which isconnected by a push-pull rod 26 to a whiflletree 2B. The whifiletree 28is connected at its inner end to amovable plate 30 which can be moved upand down to change the pitch of all of the blades [8 by total pitchcontrol means, not shown. The outer end of the whiflletree 28 isconnected by push-pull rod 32 to a plate to which is driven around withthe shaft [4. The plate 34 is connected by suitable bearings with anon-rotatable tilt plate 315 which can be tilted in any direction inazimuth by a plurality of push-pull rods 40, only one of which is shown.When the rod 40 is moved up or down, the plates 34 and 3B are tiltedcorrespondingly and the pitch of the blades [8 Will be controlledcyclically in accordance with such tilting. When the rod 40 is in a midor neutral position, the blades maintain the same pitch during an entirerevolution.

The rod 40 is positioned by a crank 42 pivoted upon the framework of thebody Ill, and is moved up and down by a rod 44 connected at its lowerend to a bell crank 46 pivoted by its mid pivot to a fixed portion ofthe craft. The bell crank 46 is rocked by a rod 48 which in turn ismoved by a control stick 50 in the occupants portion of the body It. Asthe control stick 50 is pushed forwardly, the rod 40 will be movedupwardly through the linkage above described, and as the control stick50 is moved 'backwardly, the rod is pulled down. With the stick 50 heldin the neutral position shown, the rod 40 will likewise remainsubstantially in the position shown in which the blades I-B have aconstant angle of incidence through their entire revolution. Lateralcontrol is effected by similar linkage, not shown.

When the rotor blades it are turned by the engine I2, or due toaerodynamic action in the autorotation state, the control rod 4!] willbe subjected to cyclical vibrations if the pitch of the rotor blades itdiffer with respect to each other, or if the aerodynamic pitchingmoments of these blades are difierent. In order to measure the differentaerodynamic pitching moments of the several blades, an indicatingmechanism is provided as generally indicated at 52 in Fig. 2. Thismechanism 52 comprises a chart 54 that is driven by and may be securedto the drive shaft 14 and a stylus arm 56 mounted upon the pushpull rod44 by a pivot bracket 58. The stylus arm 55 is equipped with a markingdevice, or stylus, til at its outermost end for marking the chart 54.The arm 56 is pivoted at a point between its ends by a multiplyingmechanism comprising a slip friction device including a spring 32cooperable with a clip 64 carried by a pivot 65, which spring and cliprestrain the pivot to a position along a Wire 68 that is secured at itsupper and lower ends to fixed parts of the body of the helicopter. Theslip friction device may be moved along the wire 68 so that the markingpoint Gil of the stylus arm 56 may register with a fixed mark around thechart 54, or to a position in which new marks may be made upon the chart5d, and allows for movement of the controls.

As best shown in Fig. 2, each blade when in the position shown will haveits push-pull rod 32 in substantial registry with the push-pull rod 40at one position in its cycle, so that any net difference of the pitchingmovements of the three blades at this time will be transferred throughthe rod 50 to therod M to cause the stylus arm 56 to be moved. Theresulting chart when utilized as set forth hereinafter, permitsappraisal of the pitching moments of all the blades and, therefore, itis necessary to use but one indicating mechanism 52.

In Figs. 4 and 5, the charts 54 are shown as they would appear ifunrolled and after marks have'been made thereon by the marking point 60of the stylus arm 56. As the shaft I4 moves through a completerevolution from to 360 on the chart, a push-pull rod 32 of a blade linesup with control rod 60 at the 0", 120, 240 and 360 positions on thechart. In Fig. 4, a straight line marked stick-locked could be made byholding the control stick by a suitable fastener and operating the rotorblades I8 and placing the marking point 60 against the chart 54. Undersuch conditions, any vibration of the rotor blade cannot be transferredto the marking point 60 because of the locked stick. The line markedstick free is irregular and indicates variations in the pitching momentsof the several blades between zero degrees and 360 degrees on the chart.

At the zero position on the chart 54, the line marked stick freeindicates that there is a positive pitching moment of the blade I8 whichhas its control linkage in registry with the rod 40. Inasmuch as thereare three blades I8 spaced substantially 120 degrees apart,substantially 120 degrees later, the indication of the stick free lineon the chart 5% at 120 degrees will indicate positive pitching momentfor the second blade of the three. 120 degrees still later, the thirdblade I8 will have its control linkage in registry with the push-pullrod 50 and the chart indicates a negative pitching moment.

Upon examining a chart as described above, an operator will be apprisedof the fact that the first and second blades have a positive pitchingmoment, with the second blade having a slightly greater moment than thefirst blade. Each blade may be marked with a distinguishing color and, v

if desired, the lines denoting the degree position upon the chart may beof the same color, and the operator, by moving a blade to a position inwhich its control linkage is in registry with pushpull rod 40, will thenknow that the corresponding line on the chart beneath the marking point60 should correspond to this color. With such information, the operatorwill then be able to bend the tab of the first and second bladesdownwardly to a required degree to give a more negative pitching momentto such blades. The third blade shows a negative pitching moment so thatits tab 20 will need be bent upwardly, but to a lesser extent than theother two tabs were bent downwardly because this moment is of a lowermagnitude than the positive moments of the first two blades. Thereafteranother run may be made and a new tracing made by the marking point 60upon the chart 54.

Fig. 5 discloses what is considered to be a desirable trace of theresulting moments of all blades as recorded with proper trim tabadjustment thereof and the stick free. The line bearing the legend tabadjusted-no resulting moment is substantially horizontal and the irregu-4 larity thereof is: caused only by engine vibration due to explosions,and gear tooth vibrations in the reduction gearing I6.

The indicating mechanism 52 of this invention is of such simpleconstruction that it may be made part of the regular equipment for thehelicopter so that periodically it may be used to correct the bladeaction. When not in use the slip friction device may be slid along theWire 68 so that the stylus is out of the way of other operating parts ofthe machine, or the entire mechanism can be easily removed and replaced,if desired. It is also obvious that the chart 54 might be operated bydifferent parts of the mechanism associated with the rotor drive andthat the stylus could be actuated from other of the rods connectedbetween the control stick 50 and the tilt plate 36, and that the choiceof position for this mechanism may be left with the mechanic who is touse the device.

It will be understood that in testing the rotor of a helicopter as abovedescribed the helicopter would be held stationary 0n the ground eitherby loading it with sufiicient weight or by using tiedown cables.

In Fig. 6, a modification of the indicating mechanism for performing thefunctions outlined above is shown and comprises a drive shaft I I4,which may be the rotor drive shaft I4 of Fig. 2, mounting a plate II6which turns cam projections I I8, one of which is shown in engagementwith a breaker switch generally indicated at I20. The breaker switch I20engages three times in each revolution of the rotor for a threebladedrotor at the time that the rotor control mechanism is in registry with acontrol rod I 14 corresponding to rod 44 of Fig. 2. At the time ofengagement of the switch I20 the force in the control rod I44 due to theaction of a rotor blade will be reflected in a strain gauge I46, or thelike, which registers the strain in the rod I44. Obviously, otherpressure responsive electrical means could also be used to generate asignal. Wires from the breaker switch I20 and the strain gauge I46 leadto a suitable recorder I25 which may carry a chart I26 driven by theshaft I IE or otherwise. Current for the circuits to the breaker switchI20 and the strain gauges I45 are supplied from suitable sources, notshown, through wires I28. The breaker switch I20 may energize a springbiased solenoid I30, or the like, to move a stylus I32 to make a markI34 on the chart I26. Peaks I36 on the mark I3 3 will occur degreesapart upon energization, of the solenoid I30 by the breaker switch I20under the influence of the cams H8. 1

To obtain a trace upon the chart I26, of the force caused by bladeaction, the strain gauge I 36 may operate a solenoid I40, or the like,to make a mark I42 upon the chart I26. The mark I42 will accuratelyindicate upon the chart the force upon the control rod I44. With suchstructure, by providing an external driving means for chart I26 thechart and mechanism containing the chart may be removed from the body ofthe heli copter so that a ready field test may be obtained merely byplugging into the circuits of the breaker switch I20 and the straingauge I35. Obviously, electrical equivalents could be used for thevarious parts of the combination to obtain the same result as obtainedby the means explained.

While we have shown and described two modifications of our invention andthe method for using the same, it will be understood that modified meansand methods will occur to those skilled in the art within the spirit andscope of the following claims:

We claim:

1. Apparatus for testing helicopter rotors, in combination, a driveshaft, a plurality of variable pitch blades driven by said shaft, arotating pitch control mechanism driven by said shaft and connected tosaid blades, a non-rotating pitch control mechanism connected to saidrotating mechanism for controlling the pitch of said blades, a movablechart, a stylus for making a trace on said chart, means responsive tovibrations and the dynamic pitching moment of each blade as it passes agiven point during each revolution, said last mentioned means beingconnected to said non-rotating control mechanism, means operativelyconnecting said responsive means and said stylus so that said stylusmarks and records on said chart the combined pitching moments for all ofsaid blades, and means including an element driven by said shaft forindicating on said chart the points on the trace of said stylus at whicheach of said blades passes said given point.

2. Apparatus for testing helicopter rotors, in combination, a driveshaft, a plurality of variable pitch blades driven by said shaft, arotating pitch control mechanism driven by said shaft and connected tosaid blades, a non-rotating control mechanism connected to said rotatingmechanism for controlling the pitch of said blades, a movable chart, astylus for making a trace on said chart, means responsive to thecombined dynamic pitching moments of said blades as they pass a givenpoint during each revolution, said means being connected to saidnon-rotating control mechanism, means operatively connecting saidresponsive means and said stylus so that said stylus marks and recordson said chart the combined pitching moments of said blades, and meansincluding an element driven by said shaft for indicating on said chartthe point on the trace of said stylus at which each of said bladespasses said given point.

3. Apparatus for testing helicopter rotors, in combination, a shaft, aplurality of blades mounted on said shaft for pitch varying movement,means for varying the pitch of said blades including a swash platemechanism having relatively stationary and rotatable parts, controlmembers connecting the pitch varying elements of each of said bladeswith said rotatable part, means including a pilot operative memberconnected to said relatively stationary part, and recordin means forregistering forces set up in said pilot operative member and in theconnections between the latter and said stationary part by the dynamicpitching moments of the several blades as the control member associatedwith each blade moves past said pilot operative member including, amovable chart, a stylus for marking said chart, means carried by saidpilot operative memher and responsive to the combined dynamic pitchingmoments of said blades, means operatively connected to said lastmentioned means and to said stylus for moving the latter to mark saidchart in proportion to the magnitude of said pitching moments, and meansincluding an element driven by said shaft for indicating on said chartthe points on the trace of said stylus at which said control memberspass said pilot operative member.

4. Apparatus for testing helicopter rotors, in combination, a shaft, aplurality of blades mounted on said shaft for pitch varying movement,means for varying the pitch of said blades including a swash platemechanism having relatively stationary and rotatable parts, generallyupright members connecting the pitch varying elements of each of saidblades with said rotatable part, means including a generally uprightpilot operative member connected to said relatively stationary part, andrecording means for registering forces set up in said pilot operativemember by the pitching moments of the several blades as the uprightmember associated with each blade moves past said pilot operative memberincluding, a movable chart, a stylus for marking said chart, meanscarried by said pilot operative member and responsive to the forcesexerted on said member due to the combined pitching moments of saidblades, means operatively connected to said last mentioned means and tosaid stylus for moving the latter to mark said chart in proportion tothe magnitude of said pitching moments, and means including an elementdriven by said shaft for indicating on said chart the points on thetrace of said stylus at which said upright members pass said pilotoperative member.

5. Apparatus for testing helicopter rotors, in combination, a shaft, aplurality of blades mounted on said shaft for pitch varying movement,means for varying the pitch of said blades including control mechanismhaving relatively stationary and rotatable parts, control membersconnecting the pitch varying elements of each of said blades with saidrotatable part, means including a pilot operative member connected tosaid relatively stationary part, and recording means for registeringforces set up in said pilot operative member by the pitching moments ofthe several blades as the control member associated with each blademoves past said pilot operative member including, a movable chart, astylus for marking said chart, means carried by said pilot operativemember and responsive to the forces exerted on said member due to thecombined pitching moments of said blades, means operatively connected tosaid last mentioned means and to said stylus for moving the latter tomark said chart in proportion to the magnitude of said pitching moments,and means including an element driven .by said shaft for indicating onsaid chart the points on the trace of said stylus at which said controlmembers pass and pilot operative member.

ANTON R. HOFFMANN. WALTER J. CRUMP.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 815,303 Mason Mar. 13, 1906981,989 Fitch Jan. 17, 1911 1,316,260 Zahm Sept. 16, 1919 1,457,629Lawaczeck et a1. June 5, 1923 2,151,728 Baker Mar, 28, 1939 2,166,932Keinath July 25, 1939 2,252,464 Kearns et al. Aug. 12, 1941 2,291,475Kellogg et a1 July 28, 1942 2,343,383 Martin et a1 Mar. 7, 19442,361,990 Brown Nov. 7, 1944 2,380,582 Cierva July 31, 1945 2,385,881Peterson Oct. 2, 1945 2,415,148 Sikorsky Feb. 4, 1947 2,451,541 DomanOct. 19, 1948

